ABSTRACT: The systematic change in a trait with size is a concise means of representing the diversity and organization of planktonic organisms. Using this simplifying principle, we investigated how interactions between trophic levels, resource concentration, and physiological rates structure the planktonic community. Specifically, we used 3 size-structured nutrient-phytoplankton-zooplankton models differing in their trophic interactions, ranging from herbivorous grazing on one size class to omnivorous grazing on multiple size classes. We parameterized our models based on an extensive review of the literature. The maximum phytoplankton growth, maximum microzooplankton grazing, and phytoplankton half-saturation constant were found to vary inversely with size, and the nutrient half-saturation constant scaled positively with size. We examined the emergent community structure in our models under 4 nutrient regimes: 10, 20, 25, and 30 µM total N. In all models under all nutrient conditions, the normalized biomass of both phytoplankton and microzooplankton decreased with increasing size. As nutrients were increased, phytoplankton biomass was added to larger size classes with little change in the extant smaller size classes; for microzooplankton, spectra elongated and biomass was added to all size classes. The different grazing behaviors among models led to more subtle changes in the community structure. Overall, we found that phytoplankton are top-down controlled and microzooplankton are largely bottom-up controlled. Sensitivity analyses showed that both phytoplankton and microzooplankton biomass vary strongly with the size-dependence of the maximum grazing rate. Therefore, this parameter must be known with the greatest accuracy, given its large influence on the emergent community spectra.